1. Field of the Invention
The invention relates to a control apparatus and control method for an automatic transmission, a program for directing a computer to execute the method, and a storage medium on which the program is recorded. More particularly, the invention relates to technology for correcting the hydraulic pressure supplied to a friction engagement element during a gear shift.
2. Description of the Related Art
Automatic transmissions establish a gear speed by engaging a friction engagement element operated by hydraulic pressure. Such automatic transmissions have individual differences in terms of dimensions and characteristics. Therefore, for example, during a gear shift, hydraulic pressure according to control is not always supplied to the friction engagement element. If hydraulic pressure according to the control is not supplied to the friction engagement element, it may result in large shift shock or poor shift response. Therefore, the hydraulic pressure supplied to the friction engagement element is corrected based on the state of the automatic transmission, such as the input shaft rotational speed of the automatic transmission, during a gear shift.
When the automatic transmission shifts, control to suppress torque output from the power source, for example, may be performed simultaneously. In this case, the state of the automatic transmission during the gear shift may change irrespective of the hydraulic pressure supplied to the friction engagement element. As a result, the hydraulic pressure may be corrected erroneously. To avoid this, technology has been proposed that prohibits a hydraulic pressure correction when the state of the automatic transmission may change due to a different external factor than the hydraulic pressure supplied to the friction engagement element.
Japanese Patent Application Publication No. JP-A-8-285065 describes a shift control apparatus for a vehicular automatic transmission that is provided with a learning portion. This learning portion corrects the hydraulic pressure when releasing a hydraulic type friction engagement device that is released to suppress the amount of overshooting of the engine speed that is generated during a clutch-to-clutch shift in which a predetermined hydraulic friction engagement device, from among a plurality of hydraulic friction engagement devices provided in the automatic transmission, is released while at the same time another hydraulic friction engagement device is engaged. The shift control apparatus also includes a vehicle-state determining portion and a learning-correction prohibiting portion. The vehicle-state determining portion determines whether the vehicle is in a preset reference running state. The learning-correction prohibiting portion prohibits a learning correction by the learning portion when the vehicle state determining portion has determined that the vehicle is not in the reference running state.
According to the shift control apparatus described in Japanese Patent Application Publication No. JP-A-8-285065, a learning correction is prohibited when it is determined that the vehicle is not in the preset reference running state. As a result, a learning correction is prohibited when the vehicle is running in a state which may affect the amount of overshooting of the engine speed that is generated during the clutch-to-clutch shift under fixed conditions, for example. Therefore, learning control based on an erroneous overshooting amount, including the amount of fluctuation due to external factors, is not performed.
However, if a correction of the hydraulic pressure is prohibited while the vehicle is running in a state that may affect the amount of overshooting of the output shaft rotational speed of the power source, i.e., while the vehicle is running in a state in which the state of the automatic transmission may change, as in the shift control apparatus described in Japanese Patent Application Publication No. JP-A-8-285065, the opportunities to correct the hydraulic pressure are that much fewer. In recent years, there has been a trend to increase the number of gear speeds in automatic transmissions, and if there are fewer opportunities to correct the hydraulic pressure, the number of shifts in which there is no opportunity to correct the hydraulic pressure will increase.